American Board of Ophthalmology (ABO) Practice Exam 2025 – The Complete All-in-One Guide to Exam Success

A positive Marcus Gunn pupil test indicates a relative afferent pupillary defect (RAPD). This test is often performed when assessing the function of the optic nerve. When a light is swung between the eyes, if one pupil constricts less than the other in response to the light stimulus, it indicates a defect in the afferent pathway for that eye. This is typically due to damage or dysfunction in the optic nerve or severe retinal pathology.

In contrast, normal optic nerve function would not produce any relative afferent pupillary defect, as both pupils would respond appropriately to light stimuli. Increased intraocular pressure is associated with conditions such as glaucoma and does not specifically indicate a Marcus Gunn pupil. Lastly, a posterior vitreous detachment may lead to symptoms like flashes or floaters but does not directly manifest as a relative afferent pupillary defect. Therefore, the identification of a positive Marcus Gunn pupil is specifically and accurately linked to the presence of a relative afferent pupillary defect.